an atom. It is beyond the reach of the senses; nor is it at all probable that science or skill will ever be able to furnish instrumental aids capable of enabling man to take cognizance of the ultimate unit of matter. It exists for man only in hypothesis. Nevertheless, the fact remains, that in all the wide range of human investigation there is not a more magnificent generalization, nor one more useful to mankind in its practical results, than the atomic theory. Yet there are those who doubt its abstract correctness, and labor to disprove the existence of the atom. If the ultimate object of chemical science were to demonstrate the existence of the atom, or to seize it and harness it to the uses of mankind, it might be worth while to set the chemical fraternity right by demonstrating its nonexistence. If the practice of chemistry on the basis of the theory were defective in its practical results, or failed in universal application, it would then be the duty of scientists to discard it entirely, and to seek a better working hypothesis.
The most that can be said of any scientific hypothesis is, that whether true in the abstract or not, everything happens just as though it were true. When this test of universality is applied, when no known fact remains that is unexplained by it, the world is justified in assuming it to be true, and in deducing from it even the most momentous conclusions. If, on the contrary, there is one fact pertaining to the subject-matter under investigation which remains outside the domain of the hypothesis, or which is unexplained by it, it is indubitable evidence that the hypothesis is unsafe, untrue, and consequently worthless for all practical purposes of sound reasoning. Thus, Sir Isaac Newton, after having formulated his theorem, threw it aside as worthless, for a time, upon making the discovery that the moon, in its relations with the earth, apparently did not come within the terms of his hypothesis. His calculations were based upon the then accepted estimate of the length of a degree of latitude. This estimate having been corrected by the careful measurements of Picard, Newton